sx: test -a may not be short-circuiting

I was under the false assumption that -a, like &&, was used
short-circuit semantics.  However the behaviour of -a is not specified
by POSIX.

At least on Linux using bash or dash the behaviour is not
short-circuiting.

As there's no reason to test the RHS if the LHS is false I've switched
back to [ ] && notation which is defined to be short-circuiting.

sx

@@ -12,7 +12,7 @@ cleanup() {
     # regardless of failure.
     set +o errexit
 
-    if test "$1" -a "$(ps -o comm= "$1")" = Xorg; then
+    if [ "$1" ] && [ "$(ps -o comm= "$1")" = Xorg ]; then
         kill "$1"
 
         # Send SIGKILL after 10 seconds if the xserver is taking too long to